lubricants with a radically different
chemical makeup could significantly
improve the fuel economy of cars,
according to a recent report from
researchers at Oak Ridge National
Laboratory in Tennessee and General
Motors.
These new molecules, called ionic
lubricants, could someday be used as
additives to the base oil used in motor
oil. The compounds would form only a
small fraction of the chemicals in re-
engineered versions of engine oil, but
nevertheless could “save the U.S. tens of
millions of barrels of oil annually,” said
lead researcher Jun Qu.
Engine lubricants balance engine wear
with fuel economy. The more free-
flowing an oil is, the better the fuel
economy. But the thinner the oil, the
faster an engine wears down. The key to
improving fuel economy is to improve
the anti-wear additives that supplement
base oil, explained Qu.
Unlike oil-based molecules, ionic liquids
are “molten salts” with positively and
negatively charged particles combined
in equal measure. The particles are held
together by strong forces that keep
typical salts, like the one we eat, solid at
room temperature. Unlike these familiar
salts, ionic liquids are made of
exceptionally large molecules, the
charges from their positive and negative
ends spread thin over the surface. Their
size and structure keeps them liquid at
room temperature – like oils – but their
physical and chemical properties are
unique, said Qu.
Previous work with ionic liquids had
shown their promise as improved anti-
wear compounds. But many ionic liquids
are unstable. When exposed to the
elements, the positive and negative ions
split up, the latter then reacting with
water to form highly corrosive acids.
Another problem researchers had was
getting ionic liquids to dissolve in
common motor oils. Much like
dissolving salt in oil, or mixing oil and
water, creating a true solution of ionic
liquids and base oil is hard.
But implementing an entirely new kind
of engine that uses only an ionic liquid –
no base oil – is nearly unthinkable, said
Qu. Both the looks and performance of
cars have evolved over the last fifty
years, but, for the most part one thing
has stayed almost the same – the base
oil used in engine lubricants.
Though many of these previous studies
were scientifically interesting, they were
“far from real practical use in the near
future,” said Qu.
The researchers at Oak Ridge worked
around these problems by designing an
ionic lubricant with a negative ion
formed with phosphorus rather than
fluorine; the former is much less
corrosive. Their molecule was also large
enough that positive and negative
charges were spread thin across its
surface. This enabled it to slip into
solution with oils more easily.
When added to base oil in a one percent
proportion, the new molecule made for a
substantially less viscous liquid. But
they had to test it for engine wear.
The researchers measured its
performance in industrial tests at the
General Motors’ labs, using a
dynamometer, an instrument where a
standard 2008 Cadillac SRX engine is
hooked up to sensors that measure the
engine’s performance under different
conditions.
Using the ionic lubricant in an engine
designed for standard oils was nerve-
wracking, Qu recalled.
“I was really nervous during the tests!
First, I was worried it would leak
because it’s such a thin liquid. Then, I
worried it would wear out the engine on
this million-dollar dynamometer,” he
said.
However, the ionic lubricant additive in
base oil did surprisingly well – even by
the researchers’ expectations. The re-
engineered oil met wear-and-tear
standards of current synthetic motor
oils. And it improved fuel economy by
2% in a standard fuel-efficiency test.
“In the automotive industry, even 0.5%
is an improvement, so 2% was huge,”
said Qu.
The Department of Energy’s Fuel and
Lubricants program, which funded the
research, aimed to improve the fuel
economy of vehicles with better
lubricants by 2% by the year 2015.
“It’s only 2013 and we are already near
that goal,” Qu said. He and his
colleagues are now preparing their
results for peer-reviewed publication.
“When you look at the friction and wear
characteristics in these results, the
data are very strong. They show
these materials really can improve
energy efficiency significantly,” said
Michael Lovell, who researches the
lubricant properties of ionic liquids at
the University of Wisconsin, Milwaukee.
The technologies should reduce our
usage levels of petroleum-based
products and reduce the environmental
footprint of oil usage, said Lovell. But
despite the extensive lab tests, it’s hard
to predict how these molecules will
change engine performance or fuel
economy in the real world. “Until we
start using these lubricants in vehicles,
it’s hard to tell how they will work in the
lifetime and conditions that a real car or
truck experiences,” he said. “However,
we should all be excited that these
materials exist and have the potential to
be the lubricants of the future.”
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